It is known that during the drawing process, optical fibers to be used in telecommunications are covered with one or more layers of organic material in order to protect their surfaces. The optical fiber with a standard diameter of 125 .mu.m, is coated until it reaches a diameter of about 200 .mu.m with a primary soft coating with a low coefficient of elasticity. Then, still during the drawing step a further secondary rigid coating with higher coefficient of elasticity is deposited. The choice of coating layer thickness is particularly critical, since on it depends the origin of mechanical stresses in the fiber which can give rise to microbendings. Such fiber deformation can cause a loss in the light signal transmission. By this reason such deformations have to be avoided as much as possible.
Finally, before fabricating a cable containing a plurality of optical fibers, each fibre is subjected to a further coating operation with a colored resin layer, which raises its external diameter to about 265 .mu.m. The latter coating serves to establish a code for recognizing each fiber from among those contained in the cable. The materials used for the coating are generally mixtures of acrylic monomers and oligomers, duly enriched, which after deposition are caused to polymerize by ultraviolet radiation.
Optical and mechanical fiber characteristics are highly affected by the coating characteristics, such as layer thickness and its concentricity with respect to the fiber, the polymer type, the degree of polymerization achieved, ageing, etc. Namely, an eccentricity of the colored coating layer exceeding 5% can cause nonuniformity in the strains acting on the fiber sufficient to induce microbending. These phenomena can be emphasized at low temperatures (-20.degree. C.), where most polymers, being below their vitreous transition temperatures, exert correspond with difficulty and differently from layer to layer to the internal stresses, affecting significantly the optical fiber characteristics.
Primary and secondary coating eccentricity measurement is usually carried out during fiber drawing, so as to permit a ready adjustment of the devices acting on the mutual positioning between the fiber and deposition nozzle to the coating materials in case of a concentricity error. The measurement method described e.g. in the article entitled "High-Speed Measurement and Control of Fiber-Coating Concentricity" written by D. H. Smithgall and R. R. Frazee and issued in The Bell System Technical Journal, November 1981, pages 2065-2080, consists of illuminating the coated fiber with coherent light and of examining the symmetry of obtained interference fringes.
The light radiation, emitted by a laser source operating in the visible spectrum portion, is split into two beams and by means of mirrors each beam is sent along perpendicular directions towards the fiber, perpendicularly to its axis, so as to detect concentricity errors, whatever the fiber location. The interference fringes relevant to the two orthogonal directions are displayed on two screens placed behind the fiber and are taken by two TV cameras for a further processing of the corresponding electrical signals.
A similar method, described in the article entitled "Geometrical Uniformity of Plastic Coating on Optical Fibers" written by H. M. Presby and issued in The Bell System Technical Journal, December 1976, pages 1525-1537, analyzes the interference fringes obtained on screens placed on the same side of the light source, by using light reflected by the fiber surface, instead of transmitted light.
The methods used to measure primary and secondary coating eccentricity, cannot be directly used to measure colored resin layer eccentricity, since the pigments used for coloring absorb light just in the spectral interval at which the measurement takes place, therefore no interference patterns are obtained either in transmission, or in reflection.
Nowadays the test of the eccentricity of the coloured layer can be carried out only off line, by incorporating a fiber trunk into a block of transparent resin, by sectioning the block perpendicular to the fiber axis so as to lap the fiber end face and afterwards by observing it with a microscope. This method does not obviously permit a continuous measurement of the colored layer characteristics, nor an on line adjustment of the devices provided for the resin deposition during the process.